1. Field
The present embodiments relate to a backlight.
2. Related Art
A cathode ray tube (CRT) is widely used for a television (TV), a monitor of a measuring instrument, an information terminal, or the like. However, there is currently a demand for the smaller and lighter electronic devices. Because of the large size and weight of the CRT its application in electronic devices is limited.
As a substitute for the CRT, smaller, lighter and thinner display devices have received much attention. For example, a liquid crystal display (LCD) device, a plasma display panel (PDP), and an electro luminescence display (ELD) have all been used as a substitute for the CRT.
The LCD device has advantages of low power consumption and full color reproduction as well being small and light weight. Accordingly, the LCD device is being widely used for monitors of a mobile device, a desktop computer, and a large screen TV.
The LCD device may display a desired image by individually supplying image information to pixels arranged in a matrix and controlling light transmittance of the pixels.
The LCD device is a non-illuminant display device that cannot emit light by itself. Thus, light must be provided from an outside source, so that the LCD device can display an image. The LCD device may include a backlight assembly as an outside source.
The backlight assembly is categorized into edge type and a direct type assemblies, according to an installation position (location) of a light source.
The direct type backlight assembly includes a plurality of lamps arranged on the same plane at predetermined intervals. In the direct type backlight assembly, light from each of the lamps is emitted directly to the front of the assembly.
The edge type backlight assembly includes a lamp disposed on a side of a light guide panel. Light from the lamp is made incident to the light guide panel from the side. The light guide panel converts light into surface light and illuminates the light to the front of the assembly.
The direct type backlight assembly may achieve uniform brightness and thus is widely used in the field of an LCD device having a large-screen panel.
FIG. 1 is a view illustrating a general direct type backlight assembly.
As illustrated in FIG. 1, the general direction type backlight assembly includes a plurality of lamps 5 arranged on the same plane of a cover bottom 1. The cover bottom 1 is formed of a metallic material. An optical sheet 9 is disposed over the plurality of lamps 5 at a predetermined interval from the lamps 5. The optical sheet 9 includes a diffuser sheet 9a and a prism sheet 9b. The optical sheet 9 is disposed on a bank 1a. A reflector sheet 3 reflecting light is attached on an upper surface of the cover bottom 1. The optical sheet 9 is fixed and supported by a panel guide 7. The panel guide 7 is coupled to the cover bottom 1. The optical sheet 9 is fixed and supported between the bank 1a of the cover bottom 1 and the panel guide 7.
The lamp 5 emits light by an AC voltage supplied from an inverter (not shown). For example, electrons are emitted from a cathode of the lamp 5, and the emitted electrons collide with mercury and inert gases within a glass tube of the lamp, thus exponentially increasing the amount of electrons. A current flows within the glass tube by the flow of those electrons, and the inert gases are excited by the electrons to emit UV rays. The UV rays collide with an illuminant fluorescent substance coated on an inner surface of the glass tube, thereby emitting light.
The light from the lamp 5 is diffused and condensed by the optical sheet 9 and is directed to the front.
As illustrated in FIG. 2, the backlight assembly is problematic in that a leakage current occurs when an AC voltage is applied to the lamp. For example, since the cover bottom 1 is formed of metal, the metal and the lamp 5 act as electrodes, and a material (e.g., air) filling the space between the cover bottom 1 and the lamp acts as a dielectric substance. Thus, the parasitic capacitance (i.e. C1, C2) is formed between the cover bottom 1 and the lamp 5. The parasitic capacitance (C) may be expressed by Equation 1 below.
                    C        =                              ɛ            ⁢                                                  ⁢            A                    d                                    Equation        ⁢                                  ⁢        1            where C denotes the parasitic capacitance formed between the lamp 5 and the cover bottom 1, ∈ denotes a dielectric constant of a material filling the space between the lamp 5 and the cover bottom 1, A denotes a lamp-to-cover bottom facing area (hereinafter, referred to as a facing area), and d denotes a distance between the lamp 5 and the cover bottom 1.
As understood from Equation 1, the parasitic capacitance decreases as the distance (d) becomes longer and the facing area (A) becomes smaller. Since the distance (d) and the facing area (A) are fixed and set when the backlight assembly is laid out, the parasitic capacitance (C) between the cover bottom 1 and the lamp 5 is also fixed.
The parasitic capacitance between the lamp 5 and the cover bottom 1 includes the first parasitic capacitance (C1) between each of the lamps 5 and a lower portion of the cover bottom 1, and the second parasitic capacitance (C2) between the outermost lamp 5 and a side portion of the cover bottom 1.
Thus, in the general direct type backlight assembly, the leakage current flows through the cover bottom 1 by the parasitic capacitance between each of the lamps 5 and the cover bottom 1.
Accordingly, the leakage current causes the brightness of the lamp to decrease, thus lowering the luminous efficiency of the lamp and degrading the quality of an image.